Typically, a polyimide has high mechanical strength, heat resistance, insulating properties, and solvent resistance, and thus is widely utilized as electronic materials, such as protective materials or insulating materials for liquid crystal displays, semiconductors, color filters, etc. Furthermore, the application thereof to optical communication materials such as optical waveguide materials or substrates for mobile phones is expected.
Recently, the development of the related field is astonishing, and the properties of materials suitable therefor must be continually improved. The demand for polyimide having various properties adapted for end uses, including transparency and the like as well as heat resistance and solvent resistance, is increasing.
A wholly aromatic polyimide, resulting from polycondensation of an aromatic tetracarboxylic dianhydride and an aromatic diamine, which are conventionally useful, shows a dark amber color, and becomes problematic in applications requiring high transparency. Also, since a wholly aromatic polyimide has a high dielectric constant, the use thereof as an electronic material requiring transparency and a low dielectric constant is limited.
In order to realize transparency, an alicyclic tetracarboxylic dianhydride and an aromatic diamine are subjected to polycondensation to give a polyimide precursor, which is then imidized, thereby obtaining a polyimide that is less colored and has high transparency (Japanese Examined Patent Publication No. Hei. 2-24294 and Japanese Unexamined Patent Publication No. Sho. 58-208322).
Also, a polyimide obtained using 1,2,3,4-cyclopentatetracarboxylic dianhydride (hereinafter, abbreviated to “CPDA”) as a monomer is disclosed to serve as a gas barrier for an organic electroluminescent (hereinafter, abbreviated to “organic EL”) device (Japanese Unexamined Patent Publication No. 2006-232960).
However, the polyimide thus obtained has a low degree of polymerization and has to be improved in heat resistance, and furthermore, the optical properties required thereof are not satisfactory.